No, you need a tank heater. The light could overheat/underheat the tank water, and at nighttime the temp would drop dramatically once you turn the light off for the night. Also, you cant just leave the light on then because bettas need a day/night cycle like we do :)

No, you can't. Incandescent lights produce heat, but tetter lights produce less heat, so Fluorescent and such wouldn't do.

You should get a heater. Lights only work so much, and you have to turn them off at night, which means you're cutting off your rudimentry heating source.

Honestly, heaters don't cost that much. The better and higher quality ones will cost maybe 20 dollars or so, but seeing as they are a vital part of your betta's healthy, I think it'd be worth the investment. On top of that, if you buy high quality like Marineland brand, the companies often times will guarantee that their heaters will literally last a life time, so once you buy them, you shouldn't have to buy another (unless you get another tank :D ).